1. Field of the Invention
The present invention pertains generally to threaded pipe joints and couplings and the sealing thereof. More specifically, the present invention pertains to sealing against fluid leakage through mutually engageable helical threads of threaded pipe joints and couplings.
2. Description of the Prior Art
Typically, a threaded pipe joint comprises an externally threaded male section and an internally threaded female section having mutually engageable helical threads. This type of joint is sometimes referred to as a "pin" and "box" connection. The corresponding roots and crests of the mutually engageable helical threads define a helical path through which fluids from the pipe sections frequently leak. While some leakage may be acceptable with certain uses, leakage with other uses is not desirable and is best prohibited. This is particularly true in the oil and/or gas industry where leakage may cause environmental damage and may subject workers or surrounding populus to hazardous materials and conditions. In most uses, this helical thread path of potential leakage has been sealed or attempted to be sealed by coating the threads with pipe dope in a manner well known in the industry and described by the A.P.I. (American Petroleum Insitute).
It is estimated that, in the drilling and production of oil and/or gas wells, the pipe joints of one-third of these wells, due to pressures, temperatures and fluid compositions, would leak if sealed only by pipe dope. For this reason, some type of secondary seal may be provided in a pipe joint to prevent leakage through the helical thread path. A number of seal designs have been developed including metal-to-metal shoulder seals, annular gaskets, resilient annular seals, etc. In some instances, the seal may be located out of or adjacent to the thread area while in other instances the seal may be located in the interengaging thread area of the joint.
A common practice in the prior art has been to provide a deformable annular seal which is placed in an annular recess of one of the pipe sections. When two pipe sections are made up, this annular seal is deformed between corresponding surfaces of the pipe joint to seal the joint. This type of seal frequently requires an annular recess which may partially weaken the threaded connection and requires close attention to the installation and condition of resilient seals.
A number of other types of seals have been developed for sealing such threaded joints. For example, U.S. Pat. No. 2,087,185 discloses a seal in which molten metal is introduced into the pipe joint through an aperture. U.S. Pat. No. 2,980,451 discloses a threaded pipe joint in which a ring seal of non-resilient, extrudable material is placed in an annular groove for extrusion into adjacent threaded areas. U.S. Pat. No. 4,711,474 discloses a pipe joint which is provided with a seal ring of a composite material, one of the materials of which is softer than the other. The stress in the assembled pipe joint generated by compression of the polymeric material desired for good threads is avoided by use of a softer material. U.S. Pat. No. 4,770,448 illustrates another pipe joint, which has a seal ring which is received in an inclined groove at the base of the threads in a pipe joint.
U.S. Pat. Nos. 2,474,556; 2,631,871 and 4,620,349 illustrate a rather unique approach to sealing a pipe joint in which a radial bore is machined into the thread profile of one of the pipe joints and a plug member, of a resilient material such as rubber, is placed therein. Upon making up of the pipe joint, the plug is deformed so as to be forced into the voids existing between the roots and crests of the corresponding threads in the area of the plug, the plug then providing a "dam" blocking the helical path through which fluids might otherwise leak. In some of the embodiments of these patents, fluid communication is established between the interior of the pipe sections and the bottom of the bore hole by a fluid passage so that internal pipe pressure is exerted against the plug to aid in sealing. In another embodiment, a threaded metallic plug engages the bore for manipulation externally of the pipe so that the plug may be compressed by turning of the threaded plug.
While the plug damming approach is quite unique, many problems are associated with providing a reliable and cost effective make up of the blind bore and sealing plug. One problem arises in that air may be trapped under the plug preventing proper sealing. Due to the small size of the plugs, they are very difficult, to handle and install. In addition, if the plugs are not properly sized, making up of the pipe joint may result in leakage from flank parting of the threads due to excessive forces thereon and, if the plug is not properly secured in the blind bore, the plug may fall out or be pressed out during make-up of the joint.
While the sealing methods of the prior art may be satisfactory for many pipe joints, many others, especially in the oil and/or gas industry, require more sophisticated and effective seals. Thus, efforts continue to develop a more effective seal for pipe joints, particularly in the oil and/or gas industry.